1. Field of the Invention
The present invention relates to a light-emitting device in which a pixel portion is formed with a light-emitting element including a light-emitting layer in which an electroluminescence (light-emitting) material is formed between a pair of electrodes. Specifically, the present invention relates to a light-emitting device in which a display can be performed in both front and back sides. Further, the present invention also relates to an electronic device in which both front and back sides thereof are display screens.
2. Description of the Related Art
In recent years, a light-emitting device including a light-emitting element as a self luminous type light-emitting element has been researched and developed actively. The light-emitting device is referred to as an organic EL display or an organic light-emitting diode. Such light-emitting devices have the characteristics of high response speed, low voltage, low electric power consumption driving for displaying a moving image, and therefore, draw attraction as a next generation display such as a new generation of portable phone or personal digital assistant (PDA).
A light-emitting element having an organic compound layer as a light-emitting layer has a structure where a multilayer film is laminated over a substrate. And the multilayer film has a structure in which at least one layer including a light-emitting material is formed between a pair of electrodes, namely an anode and a cathode, one of which is transparent. Electroluminescence is emitted from a light-emitting substance containing layer by applying an electric filed to the anode and the cathode. Also, in this specification, all layers provided between the cathode and the anode is referred to as a light-emitting substance containing layer as a generic term.
In many cases, the light-emitting substance containing layer has a laminate structure typified “a hole transporting layer, a light-emitting layer and an electron transporting layer.” Materials for forming the light-emitting substance containing layer are classified broadly into a low-molecular-weight (monomer) material and a high-molecular-weight (polymer) material.
A typical light-emitting element has a laminate structure in which a light-transparent conductive film formed by sputtering over a substrate is used as an anode, a light-emitting substance containing layer and a cathode made of a metal such as aluminum are laminated thereover (hereinafter, a bottom emission type element). In the light-emitting element having this structure, light is transmitted from the anode formed over the substrate. When a display having the light-emitting element with this structure as a pixel is driven in an active matrix driving to display an image, efficiency of light-emitting is extremely reduced since a driving element over the substrate, for instance, a TFT prevents light from passing through the anode.
There is also a light-emitting element having a structure in which a metal electrode that is not light-transparent is formed over a substrate as an anode and a light-emitting substance containing layer and a light-transparent conductive substance are laminated thereover as a cathode (hereinafter, a top emission type element). In this light-emitting element having this structure, light is emitted from the cathode formed over the substrate. Therefore, even when a display is performed in an active matrix driving, light is not shut out by a driving element over the substrate.
Furthermore, a light-emitting element in which light is emitted from both sides of a cathode and an anode, in other words, a structure in which one element for top emission and bottom emission are provided in one pixel (hereinafter, a dual emission type element), is also proposed in Reference 1.
Reference 1: Japanese Patent Laid Open No. 2001-332392
When a light-emitting device having a dual emission type element described in Reference 1 in a pixel portion, however, is driven to display, an image is displayed in one side and a mirror image thereof is displayed in the other side. Accordingly, there is a problem that a character image is inverted in one side.
The dual emission type element has a cathode and an anode that are each light-transparent. Thus, a background view of the anode side is seen on the cathode side, and a background view of the cathode side is seen on the anode side. Accordingly, it is necessary to appropriately provide a material having a light shielding effect for a whole back side of a display screen to be seen for the purpose of precisely acknowledging a displayed image. In this case, the displayed image cannot be seen on the both sides at the same time.
In the dual emission type element of an active matrix driving, there is a problem that a driving element of a bottom emission type element is provided in a portion of a display portion, and thus the aperture ratio in the bottom emission element region is reduced.